The present invention generally relates to an epitaxial barrel susceptor and more particularly to an epitaxial barrel susceptor including modified pockets.
Chemical vapor deposition is a process for growing a thin layer of material on a semiconductor wafer so lattice structures of the deposited material are identical to the wafer. Using this process, a layer having different conductivity may be applied to the semiconductor wafer to achieve the necessary electrical properties. Chemical vapor deposition is widely used in semiconductor wafer production to build up epitaxial layers so devices can be fabricated directly on the epitaxial layer. For example, a lightly doped epitaxial layer deposited over a heavily doped substrate permits a CMOS device to be optimized for latch up immunity as a result of a low electrical resistance of the substrate. Other advantages, such as precise control of a dopant concentration profile and freedom from oxygen are achieved. Conventional epitaxial deposition processes are disclosed in U.S. Pat. Nos. 5,904,769 and 5,769,942, which are incorporated by reference.
Epitaxial deposition takes place in a reactor. One type of reactor is called a barrel reactor or a batch reactor. A barrel reactor has a reaction chamber that houses a susceptor for holding wafers during epitaxial deposition. FIG. 1 illustrates a conventional barrel susceptor, which is designated in its entirety by the reference number 10. The susceptor 10 has generally vertical but slightly sloping faces 12. Each face 12 has two or more shallow recesses 14 aligned in a generally vertical column. Each recess 14 is generally circular and has a diameter suitable for receiving a single wafer (not shown). Each recess also has a planar surface 16 on which a back surface of the corresponding wafer rests. In most cases, the susceptor 10 is constructed of silicon-carbide-coated graphite to reduce contaminants such as iron from the graphite being released into the surrounding process gases. Conventional barrel susceptors are described in U.S. Pat. No. 6,129,048, which is incorporated by reference.
Prior to epitaxial deposition, semiconductor wafers are loaded in the recesses of the susceptor and the susceptor is lowered into a deposition chamber. The epitaxial deposition process begins by introducing a cleaning gas, such as hydrogen or a hydrogen and hydrochloric acid mixture, to a front surface of the wafer (i.e., a surface facing away from the susceptor) to pre-heat and clean the front surface of the wafer. The cleaning gas removes native oxide from the front surface, permitting the epitaxial silicon layer to grow continuously and evenly on the surface during a subsequent step of the deposition process. The epitaxial deposition process continues by introducing a vaporous silicon source gas, such as silane or a chlorinated silane, to the front surface of the wafer to deposit and grow an epitaxial layer of silicon on the front surface. The recesses of the susceptor may include orifices for simultaneously injecting hydrogen gas over a back surface of the wafer. During both steps, the susceptor rotates in the reaction chamber.
Barrel reactors are advantageous because they can simultaneously process a plurality of wafers and thus have a higher throughput, typically between six and fifteen wafers in a single run. However, one problem encountered using conventional barrel reactors is that frequently wafers processed do not meet current thickness uniformity requirements. Current thickness uniformity requirements mandate a thickness uniformity of less than about two percent and preferably, less than about one percent. Typical thickness uniformity of wafers processed in a barrel reactor using a conventional barrel susceptor is between about two percent and about five percent for a 200 millimeter (mm) diameter wafer having a ten millimeter edge exclusion. In recent years, most silicon wafers are processed in single wafer reactors to meet the stricter thickness uniformity requirements. The single wafer reactor has a much lower throughput than the barrel reactor because only one wafer is processed in a single run in the single wafer reactor.
Accordingly, there is a need for a barrel reactor susceptor capable of producing wafers meeting current thickness uniformity requirements.